For a Public Switched Telephone Network (PSTN) service, telephone numbers are tied to a physical infrastructure and known physical addresses. If a user places a 911 call from a telephone having one of these stationary telephone numbers, the physical address for the telephone can be determined by querying an Automatic Location Identification (ALI) database.
For Internet Protocol (IP) phones, the above scenario is not possible because the IP phones can be easily moved from one physical location to another. These IP phones can re-register for service wherever the IP phone can establish an IP connection. In addition, IP phones can be located as “softphones”—software-implemented phones—on IP-capable mobile devices, such as laptops and handheld devices.
Another problem is an IP phone can be assigned a traditional telephone number (e.g., NPA-XXX-XXXX) that is normally associated with a totally different number plan area (area code) and prefix from its actual physical location. The IP phone having a corresponding telephone number may not have a relationship to its corresponding number plan area and prefix for its home location even for an IP phone physically at its home location.
Yet another problem is that several different IP phones, each of which are located at different physical locations, can be registered to the same traditional telephone number. This would be a generalization of the concept of phone extensions, except that for Voice over Internet Protocol (VoIP, also known as Internet telephony), the extensions can be geographically dispersed and nomadic. Therefore, it is difficult to determine the location of each of the IP phones.